use UUIDs instead of string ids

[kremich]

I think that the code complexity shrinked with a switch to UUID ids and it will also allow for easier transition to slick. There is one potential drawback though.

Switch from strings to UUIDs makes error responses different in some cases. One example would be that where we first accepted .as[String] every string would go in and after unsuccessful parsing to UUID we would send a vnd.err response. After changing that matcher to .as[UUID] strings that won't match UUID format will simply become a 405 , not allowed, as they won't match any path. I left a TODO in two places in tests to point out if we want it this way or is there some change required.

Additionally I have created a String extending method .uuid that will convert a string to a UUID object. It is only used for testing purposes (in tests and where we add dummy data to a MemStorage). The problem with this method is that the potential exception it can throw will be only catched at runtime. I wonder if we can make compiler check all the strings using this method. I think this could be solved with macros. What do you think @jczuchnowski ?